Rachel Cartwright teaches biology at California State University Channel Islands and has studied trends in habitat use in humpback whale mother-calf pairs in Hawaiian waters.
Thursday, Feb. 21
It?s February, and romance is in the air on Maui. Warm, tropical breezes and wafting scents of fragrant island flowers beckon newlyweds to the island, while in the surrounding waters, adult humpback whales congregate.
They traveled more than 3,000 miles to the islands? protected channels, leaving behind any usable food source, but lured by the drive to breed. Arriving from the North Pacific, the Central North Pacific humpback whale uses the inter-island channels as its primary tropical breeding ground for three months of the year. Male humpback whales sing haunting songs that echo across the underwater seascape, but with a sex ratio of at least two males for each female present, females will be choosy. Males will need both guts and guile to ensure their genes get a foothold in the next generation.
Like any other field expedition, weather shapes our days on the water. Today, heavy trade winds are blowing, but the West Maui Mountains provide a broad swath of protected water stretching across the Au?au Channel.
Skipping between tour boats and tenders from a visiting cruise ship, we head out. We?re looking specifically for competitive groups, large groups of males that form frequently at this time of year. Sometimes these groups contain a female, but at other times they may be male-only groups.
Within the groups, competition ebbs and flows with bouts of calm swimming, followed by intense, physical battles, as males vie for dominance that presumably assures mating access. Breaching becomes a contact sport as males land on one another, using the barnacles along fins and rostrum as brass knuckles and producing bubble displays to demonstrate male prowess.
Groups may range in size to 10, 12 or more male whales and typically, these groups form around females without calves. But as females become pregnant and leave the breeding grounds for feeding regions, males turn their attention to females with calves. These mother-calf pairs are the focus of our work, and for these pairs, competitive groups spell trouble.
In the middle of the Au?au Channel, around 4 miles off the shoreline of West Maui, we find our first group of the day: a group of three males in hot pursuit of a female with a young calf. They travel exhaustively in loose circular routes across the channel, attracting a small entourage of whale-watching boats as males jostle each other for access to the female.
After 40 minutes of this, the group comes to a stop. The three male whales continue to circle the female while her calf perches on her snout. We drop into the water to take a closer look. Immediately, a male approaches us.
Even though we are the size of small dolphins, he stakes his ground with a wide display of flaring pectoral fins as he swims toward us. He cruises close beneath our feet and it?s a pretty clear signal. We verify the number of animals, the arrangement of group members and quickly return to the research vessel to follow at the surface as the group resumes its travel.
Although maternal females with existing calves rarely ovulate postpartum (the rate is estimated at 8.5 percent), just this slight chance of mating success is sufficient to attract multiple male groups to mother-calf pairs. For the mother and her young calf, associations with multiple male groups are costly. Large groups travel fast and swim almost continuously. Calves may get jostled, losing the benefit of the echelon position where they usually travel alongside the mother. There?s a risk of injury as males take on each other and young uncoordinated calves get caught in the fray.
But perhaps most crucially, these associations tap the otherwise finely balanced energy budgets of maternal females. Like the rest of the population, mothers are also fasting, and yet still producing vast amounts of milk for their offspring. With many months and a migration of 3,000 miles of more between them and their feeding grounds, calories are solid currency, intended for investment in the growth of the young calf and not to be spent frivolously, avoiding unwanted male advances.
In most baleen whale breeding regions, mother-calf pairs stay well out of the way of breeding adults, choosing shallower regions or even alternate bays. However, in Maui waters our research has shown that mother-calf pairs make only minimal use of shoreline areas. Whether it?s the result of crowding in inshore areas, increased levels of vessel traffic or other associated issues we?ve yet to identify, mother-calf pairs are found across the channels where they mix into the melee of adult breeding groups.
Previously, we?ve documented travel speeds and energy budgets and we know that there is a substantial increase in calorific consumption when mother-calf pairs are associated with multiple groups. Now we?re using more precise navigational equipment to see if the frequency of occurrence or duration of these groups may vary with habitat.
Our questions are numerous. Are there alternate offshore areas where the likelihood of attracting multiple male groups is reduced? Do female-calf pairs in multiple male associations head back to shore or to other alternate areas? If so, does the behavior of the group change as habitat changes? Where and when are these associations more likely to dissipate?
Using a combination of INavX software loaded onto iPads and a wonderful new Bluetooth-enabled G.P.S. data logger, called a bad elf, we collect real-time track information that can be easily converted to KML files and brought over to Google Earth and ArcView for later analysis. This allows us to extract details on areas of occurrence and group formation, speeds of travel and path sinuosity, duration of travel and persistence of groups.
In Maui waters, a majority of mother-calf pairs in fact associate with a single male whale, known as an escort. For the female, this escort may become a proxy bodyguard, deflecting unwanted male attention, however the benefit to the male in this situation is poorly understood. Maybe the slight chance of ovulation is sufficient incentive, or maybe the male is currying favor for future years.
Whatever the case may be, we have yet to determine the details, as despite more than 30 years of research above and below the water?s surface humpback whale mating has yet to be documented in this or any other breeding area.
We know from extrapolation of whaling data that gestation lasts 11 months, however birthing, too, remains undocumented. Still, calves conceived on Maui will return as young of the year the next season and paternity studies indicate that many males will be represented in the next generation.
Watching the mother-calf pair in front of us, it seems like a challenging start to life, dodging between amorous males, whale-watching vessels, ferries, cruise ships and even well-intentioned researchers. But eventually, this mother-calf pair will shake off their rowdy companions; these groups rarely persist for more than a few hours. Today, our focal mom makes a beeline for windy waters to the north of the channel, where it?s just too rough for our 20-foot boat to follow.
We fall back into the lee and find our second group of the day, a resting mother and calf pair. They too are in the middle of the channel, but left unattended, at least for now. The calf twirls and rolls at the surface, pectoral fins absently waving in the warm, tropical air, drawing a broad circle of breaths at the surface then diving back down to the female below.
Maybe the rough and tumble of the multiple male associations helps ready the calf for what lies ahead. From encounters with predators to fierce ocean conditions, the natal migration is a testing time and it?s estimated that survival across the entire natal year is around 60 percent. But bigger is better, even for calves. Large body size reduces both the cost of swimming and the likelihood of predation, and resting periods, which promote growth, are a vital component of the calf?s early regimen.
The waters of the Au?au Channel are part of the Hawaiian Islands Humpback Whale National Marine Sanctuary, but sanctuary officials are becoming increasingly concerned as the tally of vessel strikes mounts.
Vessel traffic may, or may not, be the driver that pushes these pairs out into midchannel waters to take their chances among the courtship and mating rituals of mature adults. But as strains of whale song permeate through the hull of our research vessel, it?s a mellow scene, and at least for now it?s easy to believe that there?s still romance in the rivalry of humpback whale mating.
Source: http://scientistatwork.blogs.nytimes.com/2013/03/06/mother-whales-on-the-run/?partner=rss&emc=rss
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